The explosive growth of wireless communications along with the associated applications and services that have developed as a result, require communication channels having greater bandwidths for supporting such data intensive applications. Traditionally, one of the biggest bottlenecks in data packet transmissions has occurred in the air interface between the base station and the mobile stations, the so-called radio access network.
Third generation wireless communications (3G) include many standards and techniques which try to increase packet data transmission rates. One such technology, WCDMA HSPDA (High Speed Data Packet access) is envisaged to boost the capacity of downlink data transmission channels to a level capable of supporting the most demanding of multimedia users. For example, at the time of writing it is envisaged that data rates in excess of 20 Mbps will be available for MIMO (Multiple Input Multiple Output) systems. Similar concepts are standardized for cdma2000 systems in the US. In particular within the 1XEV-DV specification (see 3GPP2, The physical layer standard for cdma2000 spread spectrum systems—Release C, TS C. S0002-C, May 2002). Similar data throughput or spectral efficiency targets are envisioned for 4G (Fourth Generation) systems.
Moreover, many modern applications require a certain QoS (Quality of Service) to be established for the application to be correctly supported. For example, a real-time streaming video application to be sent to a user will want to establish that there is a communication channel or channels available to the user which can adequately support the required bit-rate for the relevant time. In addition, the QoS targets can be negotiated between the transmitter and the receiver.
One of the characteristics central to any wireless communication system is the so-called multipath fading effect, which results in constructive and destructive effects being produced due to multipath signals. That is, a transmitted signal may develop a plurality of secondary signals which bounce off or are delayed by certain media, for example buildings, and result in multiple signal paths being created and received.
A method for dealing with such multipath effects is by introducing so-called “diversity” into the system. That is, a plurality of independent paths are created between the transmitter and receiver. These paths can be obtained for example: over time by interleaving coded bits, over frequency by the combining of multipaths in CDMA systems, and over space by using multiple transmit antennas (transmit diversity) or receive antennas (receive diversity). Transmit diversity has been achieved in known systems using open loop space-time codes like STTD (Space Time Transmit Diversity) currently in the WCDMA Release 99 and Release 4 specification (3GPP, Physical Layer General Description, TS 25.202, 2002). The advantages of various diversity techniques have been identified for 3G and even 4G wireless communication systems. Diversity techniques alleviate the effects of poor channel conditions in providing a plurality of other independent channel options.
R. Knopp and P. Humblet in an article entitled “Information Capacity and Power Control in a Single Cell Multiuser Communications” published in the Proceedings of the IEEE ICC in Seattle in June 1995, discuss the so-called “multiuser diversity” effect. Multiuser diversity makes use of the fact that in a wireless communication system having many users, each having independent time-varying fading channels, it is likely that at any point in time there is a user with a much better channel conditions than the average. Thus, by scheduling transmissions to mobile stations according to the relative strength of the channels, the overall system performance is improved.
To implement multiuser diversity in a system it is necessary that the transmitter knows the link quality between the transmitter and each of the receivers. In Frequency Division Duplex (FDD) systems this can provided with a feedback control channel, wherein each mobile station tracks its signal quality which is fed back to the base station. It is also necessary that the base station is capable of scheduling transmissions to the relevant base stations and can adapt the transmission criteria as a function of the feedback channel quality. For example, if a mobile station returns channel information that is better than for other channels, the base station scheduler will prioritise transmission over that channel next and based on the quality of the channel will decide what transmission rate to use.
HSDPA relies on link adaptation where the transmitting element has the ability of adapting the transmission parameters to compensate for changing channel conditions. The channel conditions can be estimated based on information fed back from the receiver element. In HSDPA a parameter known as the CQI (Channel Quality Indicator) provides information on the transport format (for example: the coding, modulation, etc) to be used at the base station for transmitting to the mobile station. However, the current HSDPA specification merely describes that a CQI is chosen based on channel information received or measured at the mobile station. It has to be assumed that this channel information would typically be some sort of standard channel quality measure such as the received SNR (Signal to Noise Ratio) or FER (Frame Error rate). Although the Release 4 WCDMA specification (referenced above) does support the use of closed-loop transmit diversity techniques, the Release 5 specification for HSDPA (3GPP, UTRA high speed downlink packet access (HSDPA)—overall description, TS 25.308, 2002). Nor do the Release 5 specifications describe channel allocations means for supporting such techniques.
An International patent application published on 21 Mar. 2002 with publication number WO 02/23743 describes a system for scheduling mobile stations to download data and/or to control the transmission rate from a base station to a mobile station as a function of the downlink channel conditions. The channel conditions are measured at the mobile station and then fed back to a base station for scheduling. This system describes applying random complex scaling factors to different transmitting antennas and then receiving a resulting composite signal at a receiver.
However, the system described in WO 02/23743 does not adequately address the problems encountered for allocating channels in a multiuser environment. Specifically FIG. 1 shows a system with three users 100, 102, 104. Since the scaling factors are not carefully selected, it is apparent that more than one user may be transmitting over a particular channel at an optimal power at any point in time. In FIG. 1 if the window of the scheduler was chosen to span 30 symbols it becomes apparent that this system would not be able to allocate the optimal received power for each user since each user would have part of their maximum in the scheduled interval period. The horizontal lines 106, 108 and 110 indicate the respective maximum received power for the three users 100, 102 and 104 respectively. If the scheduling window lies from symbol numbers 40 to 70 it would catch all three of the maximum received powers for each of the three users and therefore the scheduler in this system would not be able to allocate the optimal received power Rx for each user. FIG. 1 shows that each user falling within this scheduled period of transmission will have to suffer receiving at a less than optimal power Rx as shown by 112, 114 and 116 respectively.
It is an aim of the present invention to provide a method and system for increasing the throughput of a multiuser system with multiple transmitters so that multiuser diversity and channel allocation is improved.